Many types of paper handling apparatus are known, for manipulating sheets of paper by performing a sequential series of process steps on individual sheets. Such process steps involve transporting, printing, collating, accumulating, folding, etc. In many cases, it is desirable to process a number of individual sheets so as to group a plurality of such sheets together into an ordered bundle. The process of placing such sheets in order is known as collating, while the process of grouping the plurality of sheets into a neat and aligned bundle is known in the art as accumulating. Known devices that might require a mechanism for accumulating a plurality of ordered sheets include printers, faxes, photocopiers, and mail piece creation devices.
Mail piece creation devices have traditionally been the province of very large organizations, such as banks, utility companies and governments, who require an automated system for rapidly producing a large number of mail items for postal delivery to a list of receivers that might include upwards of 1 million of addressees. To achieve this level of production capacity necessitates an industrial-sized complex for producing and handling the vast quantity of individual mail items. These systems are large complex, and typically require a full-time staff of specially trained operators in order to oversee production.
More recently, there has been demand for small and medium-sized mail piece creation apparatuses for use by small and medium sized enterprises (SMEs), on a more modest scale. To meet this demand, “desktop” sized mail piece creation devices have been offered that can be installed and operated relatively easily in an office environment. The size of these machines can vary quite considerably from a floor-standing apparatus that might occupy a small room, to devices that occupy substantially the same amount of space as a typical fax machine or desktop printer. The relative size of the different machines tends to correspond directly to the degree of functionality that they are able to provide, in terms of the number of different paper processing operations that the machine can perform. The size of the machine is also often dictated by the required capacity or throughput, that is, the number of mail pieces that can be created per unit time. For example, as a rule, the larger the machine; the larger the size of documents that can typically be processed; the larger the range of options for accommodating different sizes of paper and envelopes; the larger the number of paper-processing and envelope-handling functions available; and the more documents that can be processed within a given period of time. Nevertheless, since the office space, where the mail creation device is to be installed, is charged at a premium based on the floor area occupied, the driving influence remains towards reducing the size of these mail piece creation devices, while maintaining their throughput capacity and functionality.
One of the limiting factors in reducing the size of such mail piece creation devices is the size of the sheets of paper that are to be handled. The paper must be manipulated so as to be transported through the machine along defined paper paths, without damaging the paper by tearing or creasing, etc. One particular mechanism whose size is limited in this way is the so-called “accumulator”. An accumulator receives a plurality of sheets to be formed into an aligned stack. The sheets are typically fed sequentially one-at-a-time into the accumulator, in the desired order (i.e., already collated). The size of the accumulator mechanism has thus, typically, been governed by the size of the stack of sheets to be accumulated.
Further specific considerations also apply when creating a stack of sheets of paper that are intended to be aligned at the edges of the sheets. Precise control is required to prevent the sheet edges from becoming staggered (which is known as shingling). Factors which can affect the ability to align sheets of paper in a stack are, for example, the frictional forces between adjacent sheets of paper, and the frictional force between any driving means used to drive the sheets of paper and those sheets. A precise balance is needed between these two sets of frictional forces in order to drive one sheet relative to an adjacent sheet, as well as when it is desired to transport the fully formed accumulation without losing the alignment between sheets. More specifically, in order to drive one sheet relative to another sheet, it is necessary to apply a frictional force from the driving means which will overcome the frictional forces between the two adjacent sheets. However, the driving force must not be so great that it will either damage the sheet being driven, or cause one or both of the adjacent sheets to become buckled unintentionally. Additionally, when driving an accumulation of sheets formed into a stack, so as to maintain the accumulation as an aligned unitary body, the driving or acceleration forces applied to the stack must not be so great as to overcome the frictional forces between adjacent sheets and the stack. This is apparent, since it is generally only possible to apply a force to the top and bottom sheets in a stack, with the driving force that is applied to any sheets intermediate there between only deriving from the frictional contact between adjacent sheets. Shingling of the stack occurs when these frictional forces are overcome, causing the adjacent sheets to separate and become staggered.
Furthermore, paper to be handled by such mail creation devices must be handled relatively carefully to avoid damaging the paper. For example, if the paper is forced around a bend in a paper path that is too sharp, the paper will become creased, or may adopt a permanently bowed configuration, rather than the desired flat configuration. When attempting to transport paper along a paper feed path, it is necessary either to effectively drag the paper, by applying a pulling force at the front edge of the paper, or to propel the paper along the path, by applying a pushing force to the paper substantially behind the paper leading edge. Where a pushing force is to be employed, in any given paper handling operation, it is necessary to ensure that the force needed to drive the paper against any resistance acting in the opposite direction will not exceed the capacity of the paper to resist buckling under the influence of that driving force and the resistance. Any given sheet of paper has a given column strength, which must be overcome so as to cause the paper to buckle, that is, to bow or bend out of an aligned configuration, i.e. perpendicular to the driving force. In certain paper handling applications, a buckle may be intentionally employed, for example, to effect a paper fold at a desired location. On the other hand, unintentional buckling of paper sheets can lead to either misalignment or creasing of the sheets, which would be undesirable.
One known form of accumulator provides an accumulation chamber, into which sheets are fed sequentially in the order in which they are to be accumulated in the stack. A first sheet is fed into the chamber, and driven against a stop at the end of the chamber. A driving means in the form of a friction belt is provided along one wall of the chamber, to supply the driving force. A subsequent sheet is then fed into the chamber between the driving means and the sheet already stopped in the chamber. The driving means then drives the subsequent sheet into and along the chamber, up to and against the stop, to bring it into alignment with the first sheet already stopped in the chamber. In this arrangement, the friction belt must provide a driving force which will overcome the frictional contact between adjacent sheets as each subsequent sheet is fed into the accumulation chamber and is driven past and against the previous sheet already stopped in the chamber. At the same time, the friction belt must slip relative to any sheets already stopped in the accumulation chamber, so as neither to damage the sheet relative to which it is slipping, nor to cause the sheet to buckle as it is driven against the stop. Once the accumulation has been formed, the stop is removed and the accumulated stack is released to be delivered for further processing.
An alternative form of accumulator, which overcomes the need for any relative motion between adjacent sheets that are to be accumulated, is known as a re-circulating accumulator. Such an accumulator is known from U.S. Pat. No. 6,454,255 A1 to Allen et. al., dated Sep. 24, 2002. This known accumulator is shown in accompanying FIGS. 17 and 18. The re-circulating accumulator in Allen et al. receives sheets of paper 18 that are fed sequentially into the sheet accumulator 10 at an entry point 19. The sheets are fed into gripper jaws 12, situated at a home location 21, until the sheet is pressed against a backstop 20 (see FIG. 17). A sheet so fed is then circulated around a circular device 24 by closing the gripper jaws 12 and dragging the sheet 18 by its leading edge around a circular outer perimeter 22 of the circular device 24. A sheet so circulated lies at rest on the outer perimeter 22 (see FIG. 18), held in place by the inner perimeter 27 of a circular paper guide 29 (shown as dashed lines in FIG. 18). The sheet 18 is dragged around the circular path by rotating the gripper jaws 12 that grip the sheet 18 about an axis that is aligned with the central axis of circular device 24. Subsequently fed sheets entering the entry point 19 are fed into the gripper jaws 12 between an outer jaw 34 and any sheets already forming a stack on the outer perimeter 22 of the device (see FIG. 18). The gripper jaws are circulated once for every sheet 18 fed into the sheet accumulator 10, to accumulate each sheet 18 into the stack of sheets being accumulated. When the last sheet 18 has been fed, gripper jaws 12 are rotated out of the way of the home location 21, thereby freeing the leading edge of the stack of sheets that has been accumulated. A set of driven rollers 52 then drives the stack of sheets out of the sheet accumulator 10 to a set of take away rollers 58, which propel the stack of sheets out of the sheet accumulator 10 for further processing.